Elena Guerrero

Diversity and mesoscale spatial changes in the planktonic cnidarian community under extreme warm summer conditions

Knowing the variations of planktonic cnidarians under climate change conditions is of importance due to the key role of carnivorous gelatinous zooplankton in the pelagic ecosystem. We investigated the abundance, spatial distribution, species richness and community structure of planktonic cnidarians in a temperate area, the NW Mediterranean, during the extremely warm summer of 2003 and the average summer of 2004. The surveys covered a broad and heterogeneous area. During the warm summer, the cnidarian community was half as abundant as during 2004, whereas its diversity was higher. The mesoscale spatial distribution of several species varied between the years. The structure of the community had a coastal-offshore ordination in the typical summer, but a north–south one in the warm summer when the temperature gradient prevailed instead of the bathymetry. Only hydromedusae were more abundant during the warm summer. The dominant siphonophore, Muggiaea atlantica, exhibited one of the greatest abundance decreases in 2003. Our results emphasize that exceptionally warm summer conditions could reduce gelatinous zooplankton abundance and change their latitudinal distribution and community composition. We stress the importance of mesoscale plankton surveys, including detailed taxonomic identification, as a critical tool for better understanding marine ecosystem responses to climatic variability.

Long-term changes in the planktonic cnidarian community in a mesoscale area of the NW Mediterranean

In the present work, possible long-term changes in the planktonic cnidarian community were investigated by analyzing (1) species and community spatial distribution patterns, (2) variations in abundance and (3) changes in species richness during three mesoscale surveys representative of the climatic and anthropogenic changes that have occurred during the last three decades (years: 1983, 2004 and 2011) in the NW Mediterranean. These surveys were conducted during the summer (June) along the Catalan coast. All surveys covered the same area, used the same sampling methodology, and taxonomic identification was conducted by the same team of experts. An increase in the abundance of total cnidaria was found from 1983 to 2011. The siphonophore Muggiaea atlantica and the hydromedusa Aglaura hemistoma were the most abundant species, while Muggiaea kochii presented the largest abundance increment over time. Temperature was the main environmental parameter driving significant differences in the cnidarian community composition, abundance and spatial distribution patterns among the surveys. Our results suggest that in the current climate change scenario, warm-water species abundances will be positively favored, and the community will suffer changes in their latitudinal distribution patterns. We consider it extremely important to study and monitor gelatinous zooplankton in mesoscale spatial areas to understand not only long-term changes in abundances but also changes in their spatial distributions since spatial changes are sensitive indicators of climate change.